AbeNeedsAnswers wrote:The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox, which caused only a mild illness, they are immune from smallpox.

A. which caused only a mild illness, they are immune from
B. causing only a mild illness, they become immune from
C. which causes only a mild illness, they are immune to
D. causing only a mild illness, they became immune from
E. which caused only a mild illness, they would become immune to

E

With conditional sentences, we typically need only match up the tenses .

1st conditional:
If (simple present) then simple future
If something happens then something else will happen
EXAMPLES:
If Barb sells her car, then she will buy a scooter.
If you touch Neil's feet, he will cry.

2nd conditional:
If (simple past) then (clause beginning with would)
If something happened then something would happen
EXAMPLES:
If Barb sold her car, then she would buy a scooter.
If you touched Neil's feet, he would cry.

3rd conditional:
If (past perfect) then (clause beginning with would have)
If something had happened then something would have happened
EXAMPLES:
If Barb had sold her car, then she would have bought a scooter.
If you had touched Neil's feet, he would have cried.

The English physician Edward Jenner found that if experimental subjects were deliberately infected with cowpox...
Here we have the simple past tense were infected (i.e., 2nd conditional), so we need the second half of this conditional to have a clause beginning with would
Only answer choice E has this structure.

Cheers,
Brent